Echo compensation device and 4/2-wire interface having such an echo compensation device

ABSTRACT

An echo compensation device, having a digital transverse filter, for an information transmission system is described, as well as a 4/2-wire interface having at least one such device. Accordingly, a scale value device is provided which respectively evaluates the value of the input signal and also the values of the time-delayed signals of the transverse filter and establishes a scale value as a function thereof. Furthermore, a device is provided for scaling all the signal values with the associated scale value before these are weighted with the respectively assigned filter coefficients. After addition of the weighted scaled signal values has been performed, the value of the output signal is descaled with the same scale value with which the signal values had previously been scaled.

BACKGROUND OF THE INVENTION

The invention relates to an echo compensation device, which has adigital filter in the form of a digital transverse filter having oneinput, for an information transmission system. It relates in particularto a hybrid circuit--also termed a 4-wire/2-wire interface--having suchan echo compensation device.

The general problems of such hybrid circuits are described in the book"ADAPTIVE SIGNAL PROCESSING" by B. Widrow and S. D. Stearns, 1985Prentice Hall, Englewood Cliffs, N.J., U.S.A., on pages 338 to 347 inconjunction with pages 15 to 29. In particular, FIG. 12.30 on page 344shows a block diagram of an information transmission system having anadaptive echo compensation device, of which the signal characteristicmodel is represented in FIG. 12.31 on page 345 and which is described onpages 344 to 346. A suitable FIR filter (Finity Impulse Response filter)is shown as an adaptive transverse filter having one input in FIG. 2.2on page 17 and described on pages 16 and 17. In the case of such afilter, an input signal is led via an iterative network of delayelements having a delay factor of the same size, the output signal ofone delay element being provided in each case as the input signal forthe following delay element. Both the input signal of the overallcircuits and the output signals of all the delay elements arerespectively weighted with a coefficient, and these weighted values areadded to form a joint output signal. In the case of a hybrid circuithaving an adaptive filter, the coefficients are automatically adaptedduring operation to the respective operating conditions, in particularto the line length and the reflection factor of the system. Lessconvenient hybrid circuits, which in the case of use in normallyunchangeable network regions guarantee an adaptation of the inputimpedance of the hybrid circuit to the line to a sufficient extent, usefilters having fixed coefficients tuned to the line. Fundamentals ofsuitable adaptation algorithms are explained on pages 19 ff. of theabovementioned book. Particularly favorably suited is a so-called MMSEmethod (Minimum Mean-Square Error, also LMS for Least Mean-Square),which takes account of the minimum mean-square error.

Special problems of hybrid circuits having an echo compensation deviceand their solution are treated in DE-C 31 41 502 (U.S. Pat. No.4,381,561), U.S. Pat. No. 3,633,105, U.S. Pat. No. 3,798,560, EP-A 0 448753 (U.S. Pat. No. 5,175,763) and EP-A 0 448 754 (U.S. Pat. No.5,172,411). An embodiment of such a 2/4-wire hybrid circuit and of itsmode of operation, is described in the article "A Signal-ProcessingCodec Filter for PCM Applications" by D. Vogel, E. Schmid, J. Reisingerand L. Lerach, published in Siemens Forschungs-- undEntwicklungsberichte 15 (1985) No. 5, Berlin, W. Germany on pages 253 to258. This article treats a hybrid circuit whose echo compensation devicecontains an adaptive FIR filter which with the aid of the Leaky-LMSadaptation algorithm balances the echo path on the 4/2-wire interface inorder thereby to extinguish the echo occurring on the hybrid.

U.S. Pat. No. 4,064,379 describes an echo compensation device for ahybrid circuit in which analog input signal are sampled in time-discretefashion and are convened into a digital code using the A law. The samplevalues and the coefficients are stored in shift registers and multipliedin logarithmic form by addition in a summing device to form an outputsignal value.

It is common to all the previously described hybrid circuits thatcomputational inaccuracies occur, owing to word length restriction, inthe signal processor required to realize the filter. Because of suchcomputational inaccuracies, the echo compensation device respectivelyused cannot operate with quiet input signals or low input levels.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a hybrid circuithaving an echo compensation device or to provide one such echocompensation device by means of which echo suppression is guaranteedeven in the case of low input signal levels.

In general terms the present invention is an echo compensation devicefor an information transmission system having a digital transversefilter for amplitude-discrete and time-discrete input signals. The echocompensation device has a series circuit of identical delay elements,the instantaneous input signal value and the time-delayed signal valuesrespectively being weighted with a coefficient and added in a summingdevice to form an output signal value. A scale-value device evaluatesall the respective signal values and outputs a scale value establishedas a function of this evaluation. A device is provided for scaling allthe respective signal values with this scale value. A further device isprovided for descaling the output signal value of the summing devicewith the same scale value.

The present invention is also a 4-wire/2-wire interface having at leastone echo compensation device which contains a digital transverse filterfor amplitude-discrete and value-discrete input signals.

Advantageous developments of each of the above embodiments are asfollows.

The scale-value device determines the respective maximum signal valueand outputs a scale value established as a function of this maximumsignal value.

The scale value is linearly dependent on the respective maximum signalvalue.

The digital transverse filter is an adaptive digital filter.

According to the invention, in the case of an echo compensation deviceof the generic type for an information transmission device provision ismade of a scale value device which, respectively with a timingprescribed by the operating time of the delay element, evaluates all thesignal values, that is to say both the value of the input signal and thevalues of the time-delayed signals, and establishes a scale value on thebasis of this evaluation of all the signal values. Also providedaccording to the invention is a device for scaling all the respectivelycurrent signal values with the associated scale value. In this case,scaling is normally understood to be either a division by the scalevalue or a multiplication by the scale value.

Small signal values occurring in the case of a quiet input signal areconverted by this scaling into larger signal values before they areweighted with the respectively assigned filter coefficients. Thesecalculations essential for the filter function are thus always carriedout with relatively large signal values independently of the actual sizeof the input signal value and of the corresponding time-delayed signalvalues.

According to the invention, after addition of the weighted scaled signalvalues has been performed, the value of the output signal is descaledwith the same scale value with which the signal values had previouslybeen scaled. Descaling is understood in this case as the application ofa calculation specification to a signal which cancels the application ofthe scaling calculation specification. If the scaling has been carriedout by multiplication, the descaling is therefore carried out bydivision by the multiplier of the scaling, and if the scaling was adivision specification the descaling specification is a correspondingmultiplication specification. It is particularly favorable if thescaling factor is derived from the respectively largest signal value.

In the case of a particularly simple exemplary embodiment of the presentinvention, the scale value device determines the largest signal value,the device for scaling divides all the signal values by this largestsignal value and the device for descaling multiplies the end result ofthe addition of the weighted scaled signal values by the value of thelargest signal value determined. The scale value can, however, also beselected in any other way to be proportional to the largest signal valuedetermined.

A hybrid circuit according to the invention or a 4-wire/2-wire interfaceuses a previously described echo compensation device.

BRIEF DESCRIPTION OF THE DRAWING

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawing, and in which:

The single FIGURE shows an exemplary embodiment of a 4-wire/2-wireinterface according to the invention with the use of an echocompensation device according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the exemplary embodiment shown in the single FIGURE, an analog signalsource, for example a microphone MIK, provides a signal Z via a firstanalog-to-digital converter AD1. In the exemplary embodimentrepresented, this signal Z is sent via a first digital-to-analogconverter DA1 onto an analog 2-wire line and sent back, partiallyreflected by the latter, and superimposed on a signal emitted by asignal source (not represented), to the 4-wire/2-wire interface via asecond analog-to-digital converter AD2. The output signal of theanalog-to-digital converter AD2 is led to a summing point SUM, and theoutput signal of this summing point is transmitted via a seconddigital-to-analog converter DA2 to an analog signal sink, for example aloudspeaker LS.

The digital signal Z serves as the input signal for a transverse filterhaving a series circuit of identical delay elements V1, V2, V3 . . . Vn.In this case, each delay element V1, V2 etc. respectively provides atits output the value, delayed by the period T, of the signal present atits input. The delay element V1 thus provides at its output the signalvalue Z_(k1) which corresponds to the signal value Z present before thetime T at its input, The same relationship obtains between the signalvalues Z_(k2) at the output of the delay element V2 or at the input ofthe delay element V3 as well as the signal values Z_(k3) . . . andZ_(kn). A scale value device DWN determines for each change in thesignal values Z, Z_(k1), Z_(k2) . . . _(Zkn) the respectively largestvalue of these values, and as a function thereof provides a scale valueNW at its output. All the signal values Z, Z_(k1), Z_(k2), . . . Z_(kn)are scaled, in a device MN for scaling, with the same scale value NWassigned to them. The scaled signal values are then each weighted withthe filter coefficients W0, W1, W2, W3, . . . or Wn assigned to them andthen added in a summing device ADD. The signal Y' resulting therefrom isdescaled in a device MD for descaling with the same scale value NW whichwas previously used to scale the input signals. Furthermore, thereciprocal value NW of the calculation specification which was used inthe scaling is applied in the descaling. The device MD for descalingprovides at its output an output signal Y of the transverse filter. Thisoutput signal Y is subtracted in the summing device SUM from the outputsignal of the analog-to-digital converter AD2, the aim being tocompensate in this way an echo of the signal Z which was possiblytransmitted back.

An adaptive transverse filter is used in the exemplary embodimentrepresented in the figure, and this is represented by a control tap forsetting the coefficients W0, W1, W2 . . . Wn at the output of thesumming circuit SUM.

It is preferred to realize the echo compensation devices according tothe invention by means of appropriately controlled signal processors.

The invention is not limited to the particular details of the apparatusdepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described apparatuswithout departing from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. An echo compensation device for an informationtransmission system having a digital transverse filter foramplitude-discrete and time-discrete input signals, comprising:a seriescircuit of identical delay elements, an instantaneous input signal valueand time-delayed signal values of the instantaneous input signalrespectively being weighted with respective coefficients in a weightingdevice and added in a summing device to form an output signal value; ascale-value device for evaluating all respective signal values of theinstantaneous input signal value and the time-delayed signal values andfor outputting a scale value established as a function of thisevaluation; a scaling device for scaling all the respective signalvalues with said scale value; and, a descaling device for descaling theoutput signal value of the summing device with said scale value.
 2. Theecho compensation device as claimed in claim 1, wherein the scale-valuedevice determines a maximum signal value of the instantaneous inputsignal value and the time-delayed signal values and outputs a scalevalue established as a function of this maximum signal value.
 3. Theecho compensation device as claimed in claim 1, wherein the scale valueis linearly dependent on a maximum signal value of the instantaneousinput signal value and the time-delayed signal values.
 4. The echocompensation device as claimed in claim 1, wherein the digitaltransverse filter is an adaptive digital filter.
 5. A 4-wire/2-wireinterface having at least one echo compensation device which contains adigital transverse filter for amplitude-discrete and value-discreteinput signals, comprising;a series circuit of identical delay elements,an instantaneous input signal value and time-delayed signal values ofthe instantaneous input signal respectively being weighted withrespective coefficients in a weighting device and added in a summingdevice to form an output signal value; a scale-value device forevaluating all respective signal values of the instantaneous inputsignal value and the time-delayed signal values and for outputting ascale value established as a function of this evaluation; a scalingdevice for scaling all the respective signal values with said scalevalue; and a descaling device for descaling the output signal value ofthe summing device with said scale value.
 6. The 4-wire/2-wire interfaceas claimed in claim 5, wherein the scale value device determines amaximum signal value of the instantaneous input signal value and thetime-delayed signal values and outputs a scale value established as afunction of this maximum signal value.
 7. The 4-wire/2-wire interface asclaimed in claim 5, wherein the scale value is linearly dependent on amaximum signal value of the instantaneous input signal value and thetime-delayed signal values.
 8. The 4-wire/2-wire interface as claimed inclaim 5, wherein the digital transverse filter is an adaptive digitalfilter.